This invention relates to a surface light source device, and more particularly to a surface light source device of the type in which directivity of illumination light is modified by at plate-shaped optical block. This invention is useful when applied to back lighting arrangement of a liquid crystal display and the like.
Surface light source devices of the type in which the direction of illumination light is corrected by a plate-shaped optical block are currently known and have been used in, for example, back lighting arrangement of a liquid crystal display. In a conventional surface light source device, a primary light source is disposed behind a plate-shaped optical member and, on the other hand, a light control member for modifying directivity of emitted light is disposed along an emitting surface of the plate-shaped optical member. In another conventional surface light source device, a primary light source is disposed near the side surface of a plate-shaped optical member and, on the other hand, a light control member for modifying directivity of emitted light is disposed along an emitting surface of the plate-shaped optical member. The latter device is called a sidelight-type surface light source device.
A typical primary light source to be used in a sidelight-type surface light source device is a rod-shaped light source. The rod-shaped light source is disposed along an end surface (incidence surface) of a plate-shaped optical member called the xe2x80x9clight guidexe2x80x9d. Illumination light emitted from the primary rod-shaped light source is introduced into the light guide plate from an end surface (incidence surface) of the light guide plate. The illumination light is deflected in the light guide plate and is then emitted from the major surface (emitting surface) of the light guide plate. The emitted illumination light is used in, for example, back lighting of a liquid crystal panel. Generally, since a primary light source is disposed on the side of a light guide plate, a sidelight-type surface light source device can have a thin-type structure.
Known light guide plates to be used in the sidelight-type surface light source device include a light guide plate having a substantially uniform thickness and a light guide plate having a varying thickness gradually decreasing as the distance from the primary light source increases.
FIG. 17 is an exploded perspective view of a conventional sidelight-type surface light source device. As shown in FIG. 17, a sidelight-type surface light source device 1 comprises a light scattering guide plate 2, a primary light source 3 disposed on the side of the light scattering guide plate 2, a reflection sheet 4, and an optical block 5 as a light control member. The reflection sheet 4, the light scattering guide plate 2 and the optical block 5 are placed one over another. The light scattering guide plate 2 is a light guide plate made of a conventional light scattering guiding material.
The primary light source 3 is composed of, for example, a cold cathode ray tube (fluorescent lamp) 6 and a reflector 7 in the form of a regular reflection member partially surrounding the cold cathode ray tube 6. Illumination light is supplied from an opening of the reflector 7 toward an end surface (incidence surface) of the light scattering guide plate 2.
The reflection sheet 4 is a regular reflection sheet such as a metal foil, or an irregular reflection sheet such as a white polyethyleneterephthalate (PET) film. The reflection sheet 4 serves to return illumination light which is leaked from the light scattering guide plate 2 to the light scattering guide plate 2.
The light scattering guide plate 2 in the form of a wedge-shaped-cross-section light guide plate is composed of a matrix of, for example, polymechylmetacrylate (PMMA), and light-permeable particles different in index of refraction from the matrix and diffused uniformly in the matrix. As shown in FIG. 18, which is a cross-sectional view taken along line Axe2x80x94A of FIG. 17, illumination light L comes into the light scattering guide plate 2 from its one end surface near the primary light source, namely, an incidence surface T and propagates toward the wedge-shaped end as repeatedly reflected between a surface along the reflection sheet 4 and a surface along the optical block 5. In this specification, the surface along the reflection sheet 4 is called xe2x80x9cinclined surfacexe2x80x9d, and the surface along the optical block 5 is called xe2x80x9cemitting surfacexe2x80x9d.
During this propagation, illumination light L undergoes scattering by the light-permeable particles. If the reflection sheet 4 is irregular-reflective, it also causes irregular reflection. It should be noted that the incidence angle of illumination light L gradually decreases with respect to the emitting surface as the illumination light L is repeatedly reflected at the inclined surface.
Illumination light component whose incidence angle is less than the critical angle with respect to the emitting surface is emitted from the emitting surface. Thus the illumination light L emitted from the emitting surface has a characteristics tending to be scattered due to the light-permeable particles and also due to the reflection sheet 4, if the reflection sheet 4 is irregular-reflective. It is a common knowledge that, as is shown on an enlarged scale as indicated by an arrow B in FIG. 18, the main emitting direction of this illumination light L tends to be inclined to the end. This tendency or characteristics is called a xe2x80x9cdirectional-emission characteristicsxe2x80x9d of the surface light source device or the light guide plate. Namely, the sidelight-type surface light source device 1 has a directional-emission characteristics.
The optical block 5 serves to modify this directivity. The optical block 5 may be made of light-transmission resin such as acrylic resin. The cross-sectional shape of the optical block 5 is designed in such a manner that the thickness varies complementarily with respect to the thickness of the light scattering guide plate 2. Namely, the thickness of the optical block 5 increases as the distance from the incidence surface of the optical scattering guide plate 2 increases.
The optical block 5 is has an inner surface facing the light scattering guide plate 2, and an outer surface facing in a direction opposite to the light scattering guide plate 2. A prism surface is formed on each of the inner and outer surface of the optical block 5. The prism surface is in the form of a repetition of triangular-cross-section projections each having a pair of inclined surfaces.
The rows of projections of the inner prism surface and the rows of projection of the outer prism surface extend perpendicularly to each other. The optical block 5 is oriented in such a manner that each row of projection of the inner prism surface extends parallel to the incidence surface, while each row of projection of the outer prism surface extends perpendicularly to the incidence surface.
As is shown on an enlarged scale as indicated by an arrow B in FIG. 18, the inner prism surface modifies directivity of emitted light, which is inclined from the incidence surface T toward the wedge-shaped end, into the frontal direction of the emitting surface. On the other hand, as is shown on an enlarged scale as indicated by an arrow C in FIG. 18, the outer prism surface modifies expansion of illumination light in the direction parallel to the incidence surface T. The direction from the incidence surface T toward the wedge-shaped end is hereinafter called the Y direction, and the direction parallel to the incidence surface T is hereafter called the X direction.
The light guide plate having a directional-emission characteristics the above-mentioned light scattering guide plate and a light guide plate made of transparent or semitransparent material and having a wedge-shaped or a approximately wedge-shaped cross section as well as a flat guide plate having on the incidence surface and/or rear surface a diffusion film, a matted surface, a micro lens array, etc. Also a sidelight-type surface light source device using any of these listed light guide plates, like the foregoing device, can emit illumination light to the frontal direction efficiently.
However, in the conventional sidelight-type surface light source device, as indicated by reference character D in FIG. 17, a bright-and-dark striped pattern parallel to the incidence surface tends to occur near the incidence surface. This striped pattern would deteriorates the quality of emitted light. Though bright lines and dark bands contained in the striped pattern could be reduced somehow by treating the incidence surface of the light scattering guide plate 2 and the edges of the incidence surface, such treatment does not suffice to eliminate uneven luminance.
With the foregoing problems in view, it is an object of this invention to provide an improved surface light source device of the type using a plate-shaped optical control member and more particularly a sidelight-type surface light source device that is improved so as to minimize non-uniform luminance about the incidence surface, thereby realizing high-quality illumination light output.
A surface light source device of this invention generally comprises a plate-shaped optical member, a rod-shaped light source disposed along an incidence surface of the plate-shaped optical member, and a plate-shaped optical block disposed along an emitting surface of the plate-shaped optical member for modifying directivity of illumination light emitted from the emitting surface of the plate-shaped optical member.
According to a mode (first mode) of the invention, the optical block has a directivity modifying surface on a portion of at least one of its outer and inner sides.
The partial area occupied by directivity modifying surface is defined as at least part of whole area of the outer and inner sides of the optical block except a predetermined range near the incidence surface of the plate-shaped optical member. Namely, the predetermined range near the incidence surface of the plate-shaped optical member is void of directivity modifying surface. Directivity modifying surface is in the form of a repetition of inclined surfaces which modify illumination light in directivity of light beam emitted from the emitting surface of a light guide plate.
If a reflection member is disposed about the rod-shaped light source for reflecting part of illumination light, which is emitted from the rod-shaped light source, to supply it to the light guide plate, one end of the reflection member may be pinched between the optical block and the plate-shaped optical member. And the optical block and the plate-shaped optical member may be held in a unitary form by a holding mechanism that is integrally formed with the optical block and/or the plate-shaped optical member.
According to another mode (second mode) of the invention, the optical block has on at least one of its inner and outer sides a directivity modifying surface and has a reflection member is disposed about the rod-shaped light source for reflecting part of illumination light, which is emitted from the rod-shaped light source, to supply it to a light guide member. One end of the reflection member is pinched between the optical block and the plate-shaped optical member. The optical block and the plate-shaped optical member may be held by a holding mechanism integrally formed with the optical block and/or the plate-shaped optical member.
In the first mode of the invention, since the area near the incidence surface is devoid of directivity modifying surface, various kinds of characteristics may be given to directivity-modifying-surface-free area to effectively avoid non-uniform luminance that would occur when such area is illuminated by illumination light. At that time, if one end of the reflection member is pinched between the optical block and the plate-shaped optical member, it is possible to simplify the holding mechanism of the reflection member.
Alternatively or additionally, if a holding mechanism is integrally formed with the optical block and/or the plate-shaped optical member, it is possible to hold the optical block and the plate-shaped optical member reliably in a unitary form.
In the second mode of the invention, directivity modifying surface occupies also the area near the incidence surface, and one end of the reflection member is pinched between the optical block and the plate-shaped optical member. Also in this case, although a holding mechanism may be integrally formed with the optical block and/or the plate-shaped optical member, the optical block and the plate-shaped optical member are held in a unitary form so as to pinch one end of the reflection member.